The invention relates to a method for determining the segmental volume and the electrical parallel conductance of a cardiac chamber or a blood vessel of a patient, wherein a first indicator, which influences the conductance of the blood, is injected into the bloodstream of a patient, wherein the electrical conductance in said cardiac chamber or blood vessel is measured and wherein the sum of the segmental volume of said cardiac chamber or blood vessel and the electrical parallel conductance is calculated from said conductance, as well as to an apparatus for determining the segmental volume of a cardiac chamber or a blood vessel of a patient and to a catheter for use in said method or apparatus.
Such a method and apparatus are known from the article of Baan et al in Automedica 1989, vol. 11, pp. 357–365. A so-called electrical conductance method can be carried out therewith, wherein a catheter is introduced into one of the cardiac chambers of a patient. Said catheter, which comprises several annular electrodes, provides a signal which is a measure of the sum of the electrical conductance of the blood in the cardiac chamber and the electrical conductance of the structures surrounding the blood in the cardiac chamber, which is called parallel conductance. With regard to this overall conductance the following holds true:
                              G          ⁡                      (            t            )                          =                                            Q              ⁡                              (                t                )                                      ·                                          σ                b                                            L                2                                              +                      G            p                                              (        1        )            wherein Q(t) is the volume in the cardiac chamber, L is the spacing between the annular electrodes, σb is the electrical conductivity of blood, G(t) is the sum of the measured electrical conductance values between the successive annular electrodes, and Gp is the associated parallel conductance. Parallel conductance Gp is assumed to be constant. The determination of the parallel conductance is based on the linear relation between the measured conductance G(t) and the conductivity of the blood σb. The conductivity of the blood is changed by means of an injection of a hypertonic saline solution (0.8 M/l, 5–10 ml for humans) or a hypotonic glucose solution (glucose solution 10 ml) upstream of the catheter, for example in the pulmonary artery. The parallel conductance is determined by plotting the measured conductance at minimum chamber volume (Gminimum) at the beginning of the filling stage against the measured conductance at maximum chamber volume (Gmaximum) at the beginning of the ejection stage. These values are determined heartbeat after heartbeat during the increasing (or decreasing) conductivity of the blood following the injection of the saline solution (or glucose solution). The linear regression line through said values is extrapolated to the identity line, wherein the value in the point of intersection is considered to be the value of the parallel conductance. Since this method employs extrapolation of the relation between (Gminimum) and (Gmaximum) to the identity line for only a small number of heartbeats, a small error in the determination of (Gminimum) or (Gmaximum) will lead to major errors in the determination of the parallel conductance.
The application of the conductance method in blood vessels involves an additional problem. Since blood can be considered to be a suspension of small insulating particles (blood cells) in a conductive fluid (plasma), and blood flows more quickly in the centre of the blood vessel than near the wall of the blood vessel, the blood cells will be exposed to different shearing forces during the cardiac cycle. This will cause the blood cells to change their orientation and to deform when the cardiac output increases, as a result of which the effective path length of an electron current will become smaller and the electrical conductivity of the blood will increase. The reverse takes place when the cardiac output decreases. This results in a pulsating change in the conductivity of the blood at constant cross-sectional dimensions and a constant parallel conductance. It is for this reason that the application of the conductance method for determining the cross-sectional dimensions of a blood vessel has not been accepted so far.